This investigation deals with oscillation mechanics in man, and the inference of small airway structure therefrom. The objective is to test the hypothesis that a non-invasive measurement of upper and central airway cross-sectional area vs. distance along the airway, in combination with low frequency respiratory system impedance data (Zrs), can unveil sensitive indicators of the caliber of peripheral airways of the lung. This hypothesis supposes that the influences of small airways in Zrs will be proportionally larger and more readily identified after accounting for and removing the large and variable component of the Zrs associated with the oral cavity, glottis, and trachea. The removal of these components in vivo reveals the sub-carinal respiratory system as an object of investigation. If the hypothesis is proved to be correct, then our study should aid in defining and isolating small airway function in normal and abnormal lungs. Acoustic reflections measured at the mouth will be used to measure airway cross-sectional area at all points between the lips and carina; the loudspeaker-plethysmograph method will be used to measure total respiratory system impedance (2 - 64 Hz); a new method is proposed to deduce the sub-carinal impedances from the measures above; and the self-consistency method will be used to interpret frequency dependence of sub-carinal impedance in terms of small airway structure.